The present invention relates to a hybrid integrated circuit (hereinafter referred to as hybrid IC), wherein a circuit part, such as a semiconductor element or the like, is mounted on a printed circuit board formed by printing an electrically conductive thick film on an insulation substrate.
The printed circuit board that employs the thick film conductor is widely used in the hybrid ICs, since the board is fabricated easily by screen printing a circuit with a desired pattern.
FIG. 2 shows a hybrid IC comprising a ceramic insulation substrate 1 on which patterns of Ag-Pd alloy conductors 21 and 22 are printed. On one of the thick film conductor 21, a chip 4 for a semiconductor element is mounted by means of solder 5 through a copper heat sink 3. A terminal lead 6 of a covered wire is connected by solder 5 to another thick film conductor 22. The semiconductor chip 4 is connected to the terminal lead 6 by soldering an electrode of the semiconductor chip 4 to a connecting conductor 7, which is soldered to the thick film conduct 22.
FIG. 3 shows another hybrid IC for mounting individual elements. In the figure, a semiconductor element 41 is fixed to the Ag-Pd thick film conductor 21 through the solder 5, and terminal leads 61 and 62 are soldered to the conductors 21 and 22, respectively.
In case electricity flows through the Ag-Pd conductor of the hybrid ICs as shown in FIGS. 2 and 3, the sheet resistivity of the Ag-Pd conductor is from 25 to 40 milli-ohm, which is higher than that of the copper foil. Thus, as a current becomes large, heat generation from the Ag-Pd conductor becomes large to cause a problem of temperature rising. When more than 10A of a current flows through the Ag-Pd conductor, the conductor may burn out.
If the width of the Ag-Pd conductor is increased to reduce the resistance, the area occupied by the conductor increases to increase the size. If the thickness of the Ag-Pd conductor is increased the resistance is reduced, but it increases cost since the conductor contains precious metals and printing for the conductor must be made repeatedly.
An object of the present invention is to obviate the above problems and to provide a highly reliable, small and low cost hybrid IC, wherein in case a circuit element is mounted on a printed circuit board using an Ag-Pd thick film conductor or the like and a large current flows through the thick film conductor, the thick film conductor does not burn.